I had two tabs. I clicked Learn with Locals on the first one, finished my course, then clicked Learn with Locals on the second tab and this message showed up.
I’ve never seen that one. Does your course show a number of learned words? Because if it doesn’t, you wouldn’t (shouldn’t!) be able to review anything. This would be possible if you only went through the first learning session, with the words you have seen not fully “planted” (= learned).
I think it happened because when you open several tabs, it fetches the state of your account at this exact moment, and when you finish a lesson on one tab the other one is not updated automatically. Well, that’s my theory anyway.
Since this message, I’ve refreshed the page and everything went back to normal.